A leading archaeologist and a campaigner for the protection of British museum collections refused to take part in an Oxford Union debate on the Parthenon marbles amid accusations of bias.

Michael Daley, director of Artwatch UK, pulled out of the union's debate on Sunday and Dorothy King, dubbed the Simon Schama of archaeology, was asked but refused to take part. Both claim that its organisers were trying to load it in favour of the marbles' repatriation to Greece.

Oxford Union - the self-proclaimed "most famous debating society in the world" - has hosted a number of contentious debates during its 175-year history (speakers have included Malcolm X, Richard Nixon, Gerry Adams and OJ Simpson). Last Sunday's debate on the motion "this house believe the Elgin marbles [as they are commonly referred to] should be returned to Greece" was set to continue that tradition.

However, the two advocates of keeping the marbles in Britain pulled out days before the debate claiming they were asked to present their case alongside students, against a panel of experts and following the performance of a play they feared would sway the audience.

Dr King said: "I think the current principal should resign. It was not a fair debate. The Greek government can now claim there was a proper debate at the Oxford Union, but it isn't a proper debate if you don't have equal sides. It seems like a farce to me."

Mr Daley added: "It reflects very badly on the union."

The union has denied their allegations, claiming it was a fair and balanced debate, and the result - 133 voted to return the marbles and 75 for them to be retained - reflected the outcome of similar votes held in the past.

The debate was part of a 10-day Oxford Greek Festival and followed a specially devised play, Parthenon Lost by Constantine Sandis, which explored the various arguments for and against repatriation of the marbles. Sponsors of the festival included the British Committee for the Restitution of Parthenon Marbles and the Hellenic Foundation for Culture, which supports Greek culture abroad.

Georgina Costa, president of the union, admitted that some of the event's organisation - including the selection of speakers - had been left to the festival organisers, but maintained they did not compromise the union's integrity and the debate was fair and rigorous.

"There really was no bias, no rigging, it was a fair and free vote," said Ms Costa. "Had I found any bias I would have been very, very angry. To be honest if he [Mr Daley] felt very strongly that that was the case he should have come and debated it."

Asked about the apparent imbalance between the debating teams, she said: "That happens the whole time, it was in no way an unbalanced debate. With a subject such as the Elgin marbles, the British Museum has made their arguments clear - it doesn't take them to explain that."

At the beginning of the year the British Museum repeated its line of keeping the marbles in Britain despite a new push to return them ahead of the summer's Olympics in Athens.

Tom Littler, one of the directors of the festival, said: "We are in no way contractually or financially obliged to the British Committee for the Restitution of Parthenon Marbles - they were presenting an exhibition which didn't prejudice us or the festival. It was clear that the festival was not prejudiced since I myself was speaking against the motion.

"People pull out of union debates from time to time because they don't have a contract and aren't paid. We were disappointed that Boris Johnson, then at the last minute Brian Sewell pulled out and that Michael Daley pulled out. We had heard he is a good speaker and would have made a powerful case. We were disappointed he didn't have the faith in his abilities and arguments to represent his own case."

Sunday's debate eventually took place between the ancient historian Peter Derow, archaeologist and MP Richard Allen and William G Stewart, a prominent campaigner for the return of the marbles, arguing in favour of repatriation, and the American writer, Jared Reed, and two world champion Oxford student debaters arguing against it.